so how can I know if the Installed Memory is insufficient ?
As we stated before, when page reclaiming occurs, the candidate pages in the inactive list which belong to the process address space may be paged out. Having swap itself is not problematic situation.
While swap is nothing more than a guarantee in case of over allocation of main memory in other operating systems, Linux uses swap space far more efficiently.
Virtual memory is composed of both physical memory and the disk subsystem or the swap partition. If the virtual memory manager in Linux realizes that a memory page has been allocated but not used for a significant amount of time, it moves
this memory page to swap space.
Often you will see daemons such as getty that will be launched when the system starts up but will hardly ever be used. It appears that it would be more efficient to free the expensive main memory of such a page and move the memory page to swap.
This is exactly how Linux handles swap, so there is no need to be alarmed if you find the swap partition filled to 50%.
The fact that swap space is being used does not indicate a memory bottleneck; instead it proves how efficiently Linux handles system resources.